Human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome (AIDS), encodes three enzymes, including the well-characterized proteinase belonging to the aspartic proteinase family, the HIV protease. Inhibition of this enzyme has been regarded as a promising approach for treating AIDS. Hydroxyethylamine isosteres have been extensively utilized in the synthesis of potent and selective HIV protease inhibitors. However, this modem generation of HIV protease inhibitors has created an interesting challenge for the synthetic organic chemist. Advanced x-ray structural analysis has allowed for the design of molecules that fit closely into active sites on enzymes creating very effective drug molecules. Unfortunately, these molecules, designed by molecular shape, are often difficult to produce using conventional chemistry.
The modern generation of HIV inhibitors has structural similarities in a central three-carbon piece containing two chiral carbons that link two larger groups on each side (see, e.g., Parkes, et al., J. Org. Chem., 59:3656 (1994)). In general, the chemical bond from the central part to one of the larger groups is a carbon-nitrogen bond which is usually accomplished by reacting an epoxide with an amine. 2S,3S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride is a key intermediate in the synthesis of protease inhibitors (see, e.g., U.S. Pat. Nos. 5,585,397, 5,723,490 and 5,783,701 as well as PCT International Publication No. WO 94/05639, the teachings of all of which are incorporated herein by reference). The process currently used to prepare 2S,3S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride is illustrated in Figure I.
As illustrated in Figure I, 2S,3S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride is prepared by a 4-step process starting from a commercially available (Aerojet Fine Chemicals (Sacramento, Calif.)) starting material, 2S,3S-chloromethylalcohol (2S,3S-CMA). In the first step, 2S,3S-CMA is reacted with sodium hydroxide in THF/ethanol to give the corresponding epoxide in 91% yield. In the second step, the epoxide is dissolved in toluene and reacted with excess isobutylamine at a temperature of about 75xc2x0 C. to about 80xc2x0 C. to give Compound I. Reaction of Compound I with p-nitrobenzenesulfonyl chloride (i.e., nosyl chloride) in toluene at a temperature of about 85xc2x0 C. to about 90xc2x0 C., followed by deprotecting of the Boc protecting group with aqueous HCl at a temperature of about 85xc2x0 C. to about 90xc2x0 C. gives Compound III, i.e., 2S,3S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride, in 64% overall yield.
Unfortunately, this process has a number of drawbacks. Some of the drawbacks associated with this process include high reaction temperatures which lead to the formation of side products; low solubility of intermediates in the reaction solvent which requires high processing temperatures during work-up which, in turn, leads to longer cycle times, material handling losses, etc.; and the like. As such, there remains a need in the art for an improved process for preparing 2S,3 S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride which overcomes the drawbacks associated with the currently used process. Quite surprisingly, the present invention fulfills this and other needs.
The present invention provides a new process for the preparation of 2S,3S-N-isobutyl-N-(2-hydroxy-3-amino-4-phenylbutyl)-p-nitrobenzenesulfonylamide hydrochloride, i.e., Compound III. In this new process, one of the chemical steps (i.e., the epoxide step) as well as material handling losses associated with the previously used process have been eliminated. More particularly, using the process of the present invention, the halomethylalcohol, i.e., HMA, (e.g., chloromethylalcohol) is successfully converted to Compound III directly without the need to isolate the Boc-epoxide. Numerous publications have described the synthesis of Compound I-like materials, but they all involve reaction of an epoxide with an amine. As such, the process of the present invention is the first example of a process wherein a protease inhibitor intermediate is prepared directly from the halomethylalcohol. Moreover, the process of the present invention results in a higher yield of Compound III, while not sacrificing its purity. In addition, the process of the present invention allows for the use of the lower purity HMA, thereby eliminating a purification step. Finally, it is pointed out that elimination of the epoxide step eliminates the need to isolate a toxic intermediate (i.e., a mutagen) and, thus, circumvents safety issues involved with dust explosivity of the Boc-epoxide.
Other features, objects and advantages of the invention and its preferred embodiments will become apparent from the detailed description which follows.